Model-based evaluation of critical nursery habitats for juvenile blue crabs through ontogeny: abundance and survival in seagrass, salt marsh, and unstructured bottom

Abstract

Nursery habitats confer higher food availability and reduced predation risk, thereby contributing disproportionately more individuals per unit area to the population compared with other habitats. Nursery status is inferred through evaluation of four metrics: density, growth, survival, and linkage between juveniles and adults. Moreover, organisms commonly use multiple habitats as nurseries throughout ontogeny to satisfy shifting resource requirements. To this end, we conducted manipulative and mensurative field experiments to evaluate two nursery metrics, abundance and survival, for juvenile blue crabs across multiple size classes and habitats, including structurally complex habitats -- seagrass meadows and salt marshes -- and unstructured habitat (sand flats) in the York River, Chesapeake Bay. We also considered effects of site-specific spatial orientation within the York River, seasonality, physicochemical variables, and postlarval influx. Our results showed that abundance was higher in both seagrass meadows and salt marshes relative to unstructured sand, and positively associated with turbidity and post-larval abundance. Notably, seagrass habitats harbored the highest abundances of small ([≤] 15 mm carapace width) juveniles, whereas salt marsh edge harbored the highest abundance of medium (16-30 mm carapace width) and large (31- 60 mm carapace width) juveniles. Moreover, survival was positively associated with juvenile size and structurally complex habitats relative to unvegetated controls. Seasonally, survival peaked in April, reached a seasonal minimum in August, and increased throughout fall. Finally, habitat-specific survival was dependent on spatial position: survival was elevated at upriver salt marsh and unstructured sand habitats compared to downriver counterparts. In nursery habitats of marine species, evaluation of survival, growth, abundance and ontogenetic habitat shifts has typically focused on relatively broad size ranges through the juvenile phase. Yet, ontogenetic shifts in habitat use may occur within much narrower size ranges, which has not been well studied and which is critical to the conservation and restoration of nursery habitats. We found that habitat-specific utilization rates differed by juvenile size class over a surprisingly narrow range of size, and were related to (1) the structural and biological characteristics of the nominal nursery habitats, (2) spatial gradients of environmental variables within the tributary, and (3) the likely trade-offs between growth and survival through ontogeny. Taken together, abundance and survival results indicate that seagrass meadows are key nurseries primarily for early-stage juveniles, whereas salt marshes are an intermediate nursery habitat for larger individuals to maximize growth-to-mortality ratios. Our results underscore the need to consider both habitats as critical nurseries for juvenile blue crabs throughout ontogeny.